Austenitic cr-mn-c-n steels for elevated temperature service



AUSTENITIC Cr-Mn-C-N STEELS FOR ELEVATED TEMPERATURE SERVICE Chi-Mei Hsiao, Edward J. Dulis, and Peter Payson, Pittsburgh, Pa., assignors to Crucible Steel Company of America,

Application May 31, 1957, Serial No. 662,657

8 Claims. (Cl. 75-126) This invention pertains to austenitic steels for ele- Pittsburgh, Pa., a corporation of New Jersey a 2,909,425 u Patented Oct. 20,1959

austenite; (2) chromium to impart oxidation resistance and stabilization of austenite; and (3) carbon and nitrogen to form austenite and to combine with the strengthening elements vanadium, tungsten, molybdenum and vatedtemperature service and provides new and improved steels of this type which are characterized by exceptionally high strength at temperatures up to 1200-1500 F. or higher, and which require no additions of nickel or cobalt for imparting these properties, this in marked contrast to the conventional types of so-called super alloys in which these strategically critical elements are present in substantial or preponderant amounts.

The press toward ever higher supersonic speeds for planes, missiles and the like, and toward ever higher operating temperatures for jet and internal combustion engines, has confronted the industry with a continuous demand for new and better elevated temperature materials. The best of those developed to date require, as above stated, substantial or predominant amounts of nickel or cobalt or both, so much so that the majority cannot be classed as steels but ratheras nickel and/or cobalt base alloys.

In contrast, the present invention provides a series of austenitic steels in which nickel and cobalt are present only in residual amounts and which contain as essential constituents in addition to iron, substantial amounts of manganese, chromium, carbon and nitrogen together with relatively small amounts of one or more of vanadium,

tungsten, molybdenum and columbium, within critical ranges for each and in balanced proportions such as to impart to the steel a wholly austenitic structure and outstanding elevated temperature properties as discussed below. I Fully austenitic steels are desirable because of their superior hot workingcharacteristics and superior'elevated-temperature mechanical properties as compared to those steels containing delta ferrite at elevated temperatures. Although some martensitic steels are stronger than austenitic steels at temperatures below about 1150 F., the fully austenitic steels havesuperior strength characteristics above this temperature and the strength difierence increases as the temperature increases. The compositions of the steels of this invention have been based on the ability of: (1) manganese to produce stabilized columbium to form the carbides and nitrides that increase the elevated-temperature strength.

We have found that when the composition of steel containing about (ll/0.8% C, 10/28% Mn, 12/28% Cr and 0.1/ 0.8% N, is balanced so that it is stably austenitic, as described in our copending application Serial No. 671,616, filed July 12, 1957, according to the formula percent (C+N) is equal to or greater than 0.078 (per cent Cr-12.5), the strength of the steel at elevated temperatures generally increases as the sum of C;+N increases, as shown in the following Table I.

TABLE I Efiect of C plus N on stress rupture of stably austenitic MnCr steels Comgesition Percent; Hr. Rupture Stress alance Fe (1,000 psi.) at Indicated Steel Temperatures (F.)

0 Mn Or N o+N 1,200 1,300 1,400 1,500

Nora-All test pieces were solution treated at 2100 F. 30 except W94 which was treated at 2200 F. 30 min., and quenched.

However, when the testing temperature is above about 1200 F., steels that contain large amounts of nitrogen are subject to the formation of a lamellar microstructu're (formed by a grain boundary reaction as discussedin a paper by C. M. Hsiao and E. J. Dulis, Precipitation Reactions in Austenitic Cr-MnC-N Stainless Steels, Trans. ASM, vol. 49, 1957) that lowers the creep rupture strength of the steel.

Now we have found, and this is the crux of our present invention, that by adding one or more of the carbide forming elements V, W, Mo and Cb in proper proportions to the stably austenitic Cr-Mn--C-N steel aforesaid, we can appreciably enhance the strength of the steel, not only at 1200 F., but also at 1300 to 1500 F. Indeed, this enhanced strengthening is imparted to such a degree as to make the relatively low alloy, and nickeland-cobalt-free steel of this invention comparable and even superior in stress rupture strength at 1200 to 1350 F. to the conventional types of super-alloys, high in nickel and cobalt content, as will beshown hereinafter.

- The increase in rupture strength at 1200 F. broughtabout' by additions of one or more of the aforesaid carbide forming elements is shown in the following Table II.

TABLE II Additional data on creep rupture strength over the 65,000 p.s.i.)

Composition Percent; Balance Fe Steel Hours to Elong., R.A.,

Rupture Percent Percent Mn Cr N C+N V W Mo Cb NOTE-411112951; pieces solution treated at 2100 F., 30 min. except those of W93, .92, 91, 90, and 89 which were treated at 2200 F., and quenched.

It is to be noted from the data in the above table, that the quantitative elfects of the different carbide forming elements are different. This is to be expected since these various elements have different combining weights in forming compounds with carbon and nitrogen. It is also to be noted that although appreciable strength may be obtained with a rather large addition of one of these elements, as in steels W58 and W57, equally good and even better strength may be obtained by additions of smaller 'total amounts of two or more of these elements used at the same time, as in steels W77 to W81 and W89 to W93. In the latter steels the increases in Cr along with increases of C and N over the steels of the series W55 to W70 also contributed appreciably to the increase in strength. However, from the economic viewpoint, it is preferable to use Cr, C, and N rather than the more expensive elements V, W, Mo, and Cb. It is further to be noted that although Cb by itself, as in steel W63, appears to have little efiect on the strength of the steel at 1200 -F., nevertheless when it is present in conjunction with other carbide forming elements, it has an appreciable effect in raising the strength of the steel. This is particularly true in the series W89 to W93, in which steel W93 without Cb is appreciably inferior to the other four steels which contain Cb.

On the basis of these results and from the viewpoint of economy, we prefer to use small quantities of the carbide fo'rming elements, that is, no morethan 2.5% of each, and "to use more than two of these elements at a time in the steel of this invention. We have found that when we use more than about 7% in total amount of the carbide forming elements V, W, Mo, and Cb in the CrMn-CN steel balanced so as to be stably austenitic,'considerable difliculty is encountered in forging the steel. When the Cris over about 26% in the properly balanced stably austenitic steel, thesteel is also difficult to forge.

temperature range 1200 to 1500" F. for the steels of this invention are given in Table III below.

TABLE III Stresses for rupture in 100 hours for ]5-2-4% Cr austenitic steels containing carbide forming, or strengthening, elements V Strengthening 100-Hrs. Rupture Stress Nominal Elements (Percent) (1,000 psi.) at Indicated Steel I Base 1 Temperature F.)

No. Comp.

, (Percent) V W Mo Ob 1,200 1,300 1,400 1,500

.2 1.4 .7 .7 53' 38 25 17 O 8 2 1 .9 42 27 17 12 [Mn 2 1 .9 51 as 24 15 15 or 3 1 1 51 38 25 17 4 N 4 2 .9 38 26 17 4 2 1 51 39 26 3 17 40 29 19' 13 57 42 1 29 18 56 '41 26 17 41 28 18 '60 40 28- 18 g3 g6 23 15 0 27 18 .4 C 54 40 12 M11 55 40 26 16 18 Cr 58 '41 29 18 .4 N 54 41 27 17 56 '40 '27 18 54 41 29 18 .9 58 42 28 19 1 .9 56 42 28 18 2 1, .9 '56 12v 20; .19 44 31 20 13 I 2 1.'4 .7 .7 61 44- 29' 17 .5 O 2 1 .9 61 44 29 18 12 Mn .4- 1 1 63 44 30 19 21 Or .3, 2; .9 63 16 a1 21 .5 N .3 2 1 60 42 26 15 .4 52 37 21 12 I .4 1.5. .7 66 47 26 14 .6 C .---.1 2 1 1 73 49 27' 16 12 Mn 4 s 9 71 '49 29 16 W92--- 24 Cr .4 2 .9 75 50 30 17 W93 .7 N .4 2 1 '44 24 14 W94.... 55 35 17 11 Complete analyses of the above steelsare 'given in TABLE IV Compositions of steels W71 -W76, inc.

That the values for some of these steels, particularly W80 and W92, are quite outstanding can be appreciated when the values for these steels are compared with those of the well known super-alloys. A set of curves for stress for 100 hr. rupture versus test temperature is shown graphically in the accompanying drawing. The nominal compositions of the super-alloys used for, comparison are given in the following Table V.

TABLE v 6 TABLE v11 Test Condition Results Steel No. and

Strengthening Min.

Elements Tem Stress Rupt. Elong. R.A. Creep F. (XLODO Life (Per- (Per- Rate p.s.i.) (Hrs) cent) cent) (Percent/ hr.)

1,200 65 .231 13 W90 1, 300 50 75 11 16 097 s a it a '82; 1 500 20 1s 2s 20 1:500 47 7 24 .112

1,800 7 .20 3e 40 tea 2 2 r 1 0 res 2a a 2 40V10Mo l 500 27 23 11600 10 a9 28 1,800 4 43 57 i338 2s .1 at; 3 13 0 11 V'( 1 500 20 33 11 W191 Q 11500 20 21 0 1,500 15 57 11 1,600 10 .50 23 1, 800 6 25 20 1 Complete analyses appear in Table II. v Q Norn.All samples were solution treated at 2200 F., min. water quenched.

More detailed creep rupture test data on steels of this invention are given in the following Tables VI and VII.

TABLE VI Creep and creep-rupture properties of 0.5C12Mn21 Cr--0.5N steels that contained strengthening elements Test Condition Results Steel 1 No. and

Strengthening Min.

Elements Tem Stress Rupt. Elong. R.A. Creep F. (X1,000 Life (Per- (Per- Rate p.s.i.) (Hrs) cent) cent) (Pfircent/ o ygggi rg 1, 300 40 230 18 28 .023 1, 500 20 19 27 17 W79 1 200 65 69 7 10 36 Y 1: 300 40 218 9 1, s00 20 69 23 W80 1 200 65 65 7 GIEE 11300 40 13s 9 1, 500 20 22 19 1 Complete analyses appear in Table II. Norn.-Ali samples were solution treated at 2100 F., 30 min. water quenched.

It will be, noted from the above data that all of the steels have relatively good elongation at rupture, that is, all the steels have an elongation of at least 5%.

As was pointed out above with reference to our mentioned copending application on the straight steels referred to, a stably austenitic structure is imparted by so proportioning the carbon and nitrogen in relation to the chromium content according to the formula:

(1 Percent c+N =o.07s (percent Cr-12.5)

Since, however, the steels of the present invention contain in addition to the active ferriteformer, chromium, also one or more of the additional active ferrite formers vanadium, tungsten, molybdenum and columbium, it is necessary to substitute in the above formula the total chromium equivalent of all ferrite forming elements present in the steels of the instant invention. This chromium equivalent, designated Cr is expressed by the following formula:

(2) Cr =Percent Cr+2.3% V+0.63% W -|-l.4% Mo+2.8% Cb Thus in any given composition of steel according to this invention, the chromium equivalent comprises the actual percent of chromium present in the alloys plus the weighted percents according to Formula 2 of V, W, Mo and Cb present. And for assuring a wholly austenitic structure for the steels of this invention, Formula 1 above becomes:

(3) Percent (C+N) =0.078 (Ch -12.5)

The broad and preferred ranges for the steels of this invention are:

Cb 0.5 to 1. Balance Substantially Fe.

In the appended claims the expression chromium equivalent shall have the meaning defined by the above Formula 2.

What is claimed is:

1. An alloy steel containing about: 0.2 to 0.8% carbon, 10 to 14% manganese, 14 to 26% chromium, 0.2 to 0.8% nitrogen, 0.5 to 2.5% tungsten, 0.3 to 1.5% columbium, up to 1% vanadium, up to 2% molybdenum, up to 2% silicon, balance substantially all iron, the carbon plus nitrogen content of said alloy being greater than 0.078 (chromium equivalent-12.5) and the. total content of the elements vanadium, tungsten, molybdenum and columbium present in said alloy being from about 2 to 7%, said alloy being characterized by a wholly austenitic structure and by high creep strength at elevated temperatures in excess of 1100 F.

2. An alloy steel containing about: 0.35 to 0.65% carbon, 11 to 13% manganese, 16 to 24% chromium, 0.3 to 0.7% nitrogen, 0.7 to 2% tungsten, 0.5 to 1% columbium, up to 0.5% vanadium, up to 1.5% molybdenum, 0.2 to 0.7% silicon, balance substantially all iron, the carbon plus nitrogen content of said alloy being greater than 0.078 (chromium equivalent12.5) and the total content of the elements vanadium, tungsten, molybdenum and columbium present in said alloy being from about 3 to 5%, said alloy being characterized by a wholly austenitic structure and by high creep strength at elevated temperatures in excess of 1100" F.

3. An alloy steel containing about: 0.2 to 0.8% carbon, to 14% manganese, 14 to 26% chromium, 0.2 to 0.8% nitrogen, 2 to 7% of at least one element selected from the group consisting of molybdenum, vanadium, tungsten and columbium, up to 2% silicon, balance substantially all iron, the carbon plus nitrogen content of said alloy being greater than 0.078 (chromium equivalent12.5), said alloy being characterized by a wholly austenitic structure and by high creep strength at elevated temperatures in excess of 1100 F.

4. An alloy steel containing about: 0.35 to 0.65% carbon, 11 to 13% manganese, 16 to 24% chromium, 0.3 to 0.7% nitrogen, 3 to 5% of at least one element selected .from the group consisting of molybdenum, vanadium, tungsten and columbium, 0.2 to 0.7% silicon, balance substantially all iron, the carbon plus nitrogen content of said alloy being greater than 0.078 (chromium equivalent-12.5), said alloy being characterized by a wholly austenitic structure and by high creep strength at elevated temperatures in excess of 1100 F.

5. An alloy steel containing about: 0.2 to 0.8% carbon, 10 to 14% manganese, 14 to 26% chromium, 0.2 to 0.8% nitrogen, 2 to 7% of a plurality of elements selected from the group consisting of molybdenum, vanadium, tungsten and columbium, up to 2% silicon, balance substantially all iron, the carbon plus nitrogen content of said alloy being greater than 0.078 (chromium equivalent-12.5), said alloy being characterized by a wholly austenitic structure and by high creep strength at elevated temperatures in excess of 1100 F.

6. An alloy steel containing about: 0.35 to 0.65% carbon, 11 to 13% manganese, 16 to 24% chromium, 0.3 to 0.7% nitrogen, 3 to 5% of a plurality of elements selected from the group consisting of molybdenum, vanadium, tungsten and columbium, 0.2 to 0.7% silicon, balance substantially all iron, the carbon plus nitrogen content of said alloy being greater than 0.078 (chromium equivalent-12.5), said alloy being characterized by a wholly austenitic structure and by high creep strength at elevated temperatures in excess of 1100 F.

7. An alloy steel containing about: 0.2 to 0.8% carbon, 10 to 14% manganese, 14 to 26% chromium, 0.2 to 0.8% nitrogen, 2 to 7% of a plurality of elements selected from the group consisting of molybdenum, vanadium, tungsten and columbium, including 0.3 to 1.5% columbium, up to 2% silicon, balance substantially all iron, the carbon plus nitrogen content of said alloy being greater than 0.078 (chromium equivalent-12.5), said alloy being characterized by a wholly austenitic structure and by high creep strength at elevated temperatures in excess of 1100 F.

8. An alloy steel containing about: 0.35 to 0.65 carbon, 11 to 13% manganese, 16 to 24% chromium, 0.3 to 0.7% nitrogen, 3 to 5% of a plurality of elements selected from the group consisting of molybdenum, vanadium, tungsten and columbium, including 0.5 to 1% columbium, 0.2 to 0.7% silicon, balance substantially all iron, the carbon plus nitrogen content of said alloy being greater than 0.078 (chromium equivalent12.5), said alloy being characterized by a wholly austenitic structure and by high creep strength at elevated temperatures in excess of 1100" F.

References Cited in the file of this patent UNITED STATES PATENTS 2,562,854 Binder July 31, 1951 2,698,785 Jennings Jan. 4, 1955 2,745,740 Jackson et al. May 15, 1956 FOREIGN PATENTS 152,291 Australia Jan. 25, 1938 OTHER REFERENCES Franks et al.: Trans. of the American Society for Metals, vol. 47, 1955, pages 231-266, published by the American Society for Metals, Cleveland, Ohio. 

3. AN ALLOY STEEL CONTAINING ABOUT: 0.2 TO 0.8% CARBON, 10 TO 14% MANGANESE, 14 TO 26% CHROMINUM, 0.2 TO 0.8% NITROGEN,. 2 TO 7% OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM, VANADIUM, TUNGSTEN AND COLUMBIUM, UP TO 20% SILICON, BALANCE SUBSTANTIALLY ALL IRON, THE CARBON PLUS NITROGEN CONTENT OF SAID ALLOY BEING GREATER THAN 0.078 (CHROMIUM EQUIVALENT-12.5), SAID ALLOY BEING CHARACTERIZED BY A WHOLLY AUSTENITIC STRUCTURE AND BY HIGH CREEP STRENGTH AT ELEVATED TEMPERATURES IN EXCESS OF 1100*F. 